Ultrafiltration is a process of separation of solutions or suspensions wherein a solution or suspension which contains a solute, colloidal particle or suspended particle of significantly greater dimensions than that of the solvent in which it is dissolved is fractionated by being subjected to such pressures that the solvent is forced to flow through a selected membrane. Generally, the term "ultrafiltration" may be applied to pressure-activated separations involving dissolved molecules, colloidal-sized particles or the like. Preferably, the term is applied to separations involving feed streams containing species having a molecular weight on the order of about 10 times the dimensions of the solvent molecules.
Many advantages have been realized by the employment of this relatively new separatory technique, among which may be considered reduction in time required for effecting separation, efficiency in separation, the use of generally mild operating conditions and reduced operating costs, as compared to older techniques such as evaporation, chemical precipitation, ultracentrifugation and the capability to separate species heretofore considered inseparable.
The advantages, such as mild operating conditions, are particularly important when thermally unstable or biologically active materials are to be processed.
In the employment of pressure-activated membrane separation processes, the success of operation depends, to a major degree, upon the characteristics of the membrane employed. Among the characteristics of membranes found to be desirable are:
(a) good hydraulic permeability to solvent, with the capability of transmitting liquid at high rates per unit membrane area under acceptable pressures;
(b) good mechanical durability with respect to thermal, pressure and chemical conditions of service;
(c) high fouling resistance; and
(d) capability to retain completely, or almost completely, solutes, suspended matter or colloidal particles of a molecular weight or size above a specified value.
Membranes generally finding utility in ultrafiltration techniques presently employed in the art may be classified as either (a) homogeneous, or (b) anisotropic membranes.
Homogeneous membranes have a limited utility in separatory procedures, being analogous to conventionaL filters and being non-rententive for species other than those of suspended matter or of high molecular weight. Modification of such membranes to retain correspondingly smaller molecules generally results in a corresponding decrease in permeability. Additionally, such membranes are susceptible to internal fouling by the retained species.
A form of membrane particularly desirable in separation of solutes, suspended matter or colloidal particles is the submicroscopically porous anisotropic membrane having a thin skin and an underlying supporting sponge-like backing. The skin surface of the anisotropic membrane is exceedingly thin, e.g., from about 0.1 to about 1.0 microns thickness and having an average pore diameter in the sub-micron range, for example, from about 1 to 50 millimicrons. The balance of the membrane structure is a support layer comprised of a more porous polymer structure through which fluid can pass with little or no hydrodynamic resistance. When such a membrane is employed as a "molecular filter" with the "skin side" in contact with fluid under pressure, virtually all resistance to fluid flow through the membrane occurs in the "skin" and molecules or particles having dimensions larger than the pores of the "skin" are selectively retained. Because the skin layer is of such exceptional thinness, and because the transition from the skin layer to the macroporous support layer is so abrupt, the overall hydrodynamic resistance to fluid flow through the membrane is very low and the tendency of such membranes to become internally plugged or fouled by molecules or particles is greatly reduced.
The membranes may be of various configurations such as hollow fiber, flat sheet, spiral wound sheet or tubular. Preferably, for the purposes of the present invention, hollow fiber membranes are employed.
In ultrafiltration systems now in use, the pressures which are used to produce the tangential flow of feed stream over the membrane has been limited to a low value or values, the exact pressures being governed by the nature of the membranes in resisting rupture or collapse due to applied pressure. Generally, inlet pressures are limited to about 15 to 100 psig depending on membrane configuration. Such limitations often serve to render the separatory apparatus employed in solution separation less than fully economically desirable. Ultrafiltration units or cartridges are limited in length, on the order of from about one to about four feet, due to pressure limitations, the combined positive concentrate takeoff pressure and pressure drop across the cartridge being less than the maximum pressure which can be withstood by the membrane employed. Such restrictions on the inlet pressures seriously limit the capacity of the systems to efficiently process working solutions. In order to achieve a given large rate of separation, multiple ultrafiltration cartridges must be employed in a parallel configuration. However, such an assembly results in excessively large capital and operating costs.